Various heat transfer recording methods have been known so far. Among these methods, dye diffusion transfer recording systems attract attention as a process that can produce a color hard copy having an image quality closest to that of silver halide photography. Moreover, this system has advantages over silver halide photography: it enables direct visualization from digital data; it makes reproduction simple, and the like without treatment chemicals.
Among these methods, in a sublimation type thermal transfer recording system, a heat-sensitive transfer sheet (hereinafter also referred to as an ink sheet) containing dyes is superposed on a heat-sensitive transfer image-receiving sheet (hereinafter also referred to as an image-receiving sheet), and then the ink sheet is heated, for example, by a thermal head whose exothermic action is controlled by electric signals, in order to transfer the dyes contained in the ink sheet to the image-receiving sheet, thereby recording an image information. Three colors: cyan, magenta, and yellow, are used for recording a color image by overlapping one color to other, thereby enabling transferring and recording a color image having continuous gradation for color densities.
In recent years, because an acceleration of a printer can shorten user's waiting time in the case where print is conducted in a photo shop for user's advantage, high-speed printers in the sublimation-type thermal transfer recording system, which can provide a print in a short time, have been developed and commercialized one after another.
In wide spread use of the printers in the sublimation-type thermal transfer recording system, there is a demand for a printer providing prints with good image quality under various environmental conditions without depending on an installation site. That is, in order to satisfy user's needs, it is necessary to provide a print good in image quality and free from image defect, not only under the standard air-conditioned environmental condition of offices and shops at a temperature of 23° C. to 27° C. and a humidity of 50% to 70%, but also, for example; under a high-temperature high-humidity condition in summer (e.g., temperature: 35° C., humidity: 80%) and a low-temperature low-humidity condition in winter (e.g., temperature: 10° C., humidity: 20%).
In an image-forming method by thermal transfer, two materials, a heat-sensitive transfer sheet and a heat-sensitive transfer image-receiving sheet, are used. Recently, methods of using a latex, i.e., an aqueous resin dispersion, in the receptor layer of the heat-sensitive transfer image-receiving sheet; are proposed (see JP-A-8-2123 (“JP-A” means unexamined published Japanese patent application), JP-A-2006-88691 and JP-A-2006-264092), disclosing that the heat-sensitive transfer image-receiving sheets have a suitable sensitivity and have good printing characteristics free from white spots (white spots defect in solid image).
However, these heat-sensitive transfer image-receiving sheets give images at good quality under a standard condition (e.g., temperature: 25° C., humidity: 60%), but are not resistant enough to white spot-shape image defect (hereinafter, referred to as white spots defect) after printing under a low-temperature low-humidity condition (e.g., temperature: 10° C., humidity: 20%). Further, these sheets cause other problems of image defects such as a fusion of the heat-sensitive transfer sheet and the heat-sensitive transfer image-receiving sheet after printing or a generation of separation residue lines by a discontinuous separation of the heat-sensitive transfer sheet from the heat-sensitive transfer image-receiving sheet, under a high-temperature high-humidity condition (e.g., temperature: 35° C., humidity: 80%).
On the other hand, to prevent image defects caused by a fusion between the heat-sensitive transfer sheet and the heat-sensitive image-receiving transfer sheet, a method of adding a latex polymer having a resin characteristic of a low glass transition temperature (lower than 50° C.) and a latex polymer having a resin characteristic of a high glass transition temperature (50° C. or higher) to a receptor layer is proposed (see JP-A-2007-237643). However, it is not possible by the method to prevent image defects during printing generated under a high-temperature high-humidity condition sufficiently and a degree of the image defects is remarkably increased when images are printed in high speed printers commercialized recently.
For prevention of a fusion between the heat-sensitive transfer sheet and the heat-sensitive image-receiving transfer sheet, also proposed is a heat-sensitive transfer sheet containing a particular silicon compound effective in preventing the fusion with the heat-sensitive image-receiving transfer sheet (see JP-A-4-113889). However, the application does not mention the suitability of the heat-sensitive transfer image-receiving sheet prepared by using a latex polymer, and no consideration is given to the stability of the printed image quality against a fluctuation in an environmental condition.
These conventional heat-sensitive transfer materials (heat-sensitive transfer image-receiving sheets, heat-sensitive transfer sheets) have problems that, when a continuous temperature change is applied to, the dye is transferred in an amount greater than that expected in a highlighted region (corresponding to a low-temperature region), leading to increase a fluctuation in image density in the high-lighted region and thus, causing problems such as a color disappearance (whitening) and darkening of white image, for example; of white ground of wedding dress. To solve these problems, performing a gradation adjustment by LUT during printing is known, but such a gradation adjustment is still insufficient, and thus, there exists a need for a thermal transfer material good in gradation connection.